In a motor-driven in-line screw type injection molding machine in the background art, a measuring stroke is often controlled as follows. That is, a measurement servo-motor is controlled by rotational velocity feedback control so that the rotational velocity of a screw coincides with a set value, while an injection servo-motor is controlled by pressure feedback control so that the back pressure applied onto the screw (resistant pressure against astern movement of the screw) coincides with a set value. When such rotational velocity feedback control is performed on the measurement servo-motor while such pressure feedback control is performed on the injection servo-motor, the back pressure applied to the screw is improved to coincide with the set value. However, assume that setting is done to make the back pressure zero at the end of measurement. When control is performed in this setting to make the back pressure zero at the end of measurement, there must be a variation in the position where the screw stops its astern movement, due to the injection servo-motor subjected to the feedback control giving preference to pressure.
Therefore, the present inventor has proposed a measurement control method in Japanese Patent Application No. 2003-199230. In the measurement control method, measuring operation is carried out as follows. That is, speed feedback control is performed on an injection servo-motor so that the astern speed of a screw follows an astern speed setting pattern, while pressure feedback control is performed on a measurement servo-motor so that the back pressure applied to the screw follows a back pressure setting pattern (in other words, feedback control is performed to control the measuring rotational velocity to follow the back pressure setting pattern). Thus, the position where measurement is completed is made to coincide with a set position.
In the previously proposed measurement control method, feedback control is applied to a rotational velocity command to be supplied to the measurement motor so that the back pressure follows the back pressure setting pattern. Accordingly, in the beginning of measurement, shortage of raw resin leads to failure in expected increase of the back pressure. As a result, the measuring operation may be unstable. In addition, the measuring operation may be unstable in some operating condition of a raw resin supply system or in some dry condition of the resin. This is because the feed rate of the resin fed by the screw is not always increased in spite of increase in rotational velocity of the measurement motor, with the result that the back pressure does not increase to its expected value.
Therefore, the following configuration is also conceivable. That is, in a period between the start of a measuring stroke and the middle of the measuring stroke, driving of the measurement servo-motor is controlled by open control in which the set rotation number is constant, while driving of the injection servo-motor is controlled by back pressure feedback control in which the astern speed of the screw is controlled to make a measured back pressure value coincide with a set back pressure value. On and after the middle of the measuring stroke in which the astern speed of the screw is stabilized (that is, the back pressure is stabilized), the driving of the measurement servo-motor is controlled by back pressure feedback control in which the rotational velocity of the measurement servo-motor is controlled to follow a back pressure setting pattern. Thus, the back pressure can be controlled stably by the measurement servo-motor. On the other hand, the injection servo-motor is controlled by astern speed control in which the position where the screw stops its astern movement is made to coincide with the position where the measurement is completed. When control is performed thus, the position where the measurement is completed can be controlled to coincide with its set position by the injection servo-motor, while back pressure can be controlled stably by feedback control using the measurement motor on and after the middle of the measuring stroke.
When control is performed thus, the following problem remains in spite of various advantages. That is, setting is done so that the position where the astern speed is zero in a deceleration setting pattern for the injection servo-motor in the ending of the measuring stroke coincides with the predetermined position where the measuring stroke is completed. Further, the driving of the measurement servo-motor is controlled by back pressure feedback control following a back pressure decompression setting pattern calculated into a value proportional to the deceleration setting pattern. In spite of such control, there is a problem that it cannot be guaranteed that all of a measured value of the astern speed of the screw controlled by the injection servo-motor, a measured value of the back pressure controlled by the measurement servo-motor and a measured rotational velocity of the screw turn zero concurrently at timing when the screw reaches a position where measurement is completed. The background art has showed no consideration for the control to make all of the measured value of the astern speed of the screw, the measured value of the back pressure and the measured rotational velocity of the screw zero concurrently. That is, in the background art, the deceleration setting pattern for the astern speed of the screw is not proper but is apt to be set to be steep. Accordingly, the deceleration in the rotational velocity control upon the measurement servo-motor controlling the back pressure cannot follow the control in which the astern speed of the screw approaches zero rapidly. Even when the astern speed of the screw is close to zero, the measurement servo-motor has a certain degree of rotational velocity (resin is fed at a certain rate). Thus, the back pressure is adversely increased near the zero point of the astern speed of the screw. As a result, a known suck-back operation has to be carried out after the completion of the measuring operation.